Heavy metals such as lead, zinc, mercury, silver, arsenic can be present in trace amounts in all types of hydrocarbon streams such as crude oils. The amount can range from below the analytical detection limit to several thousand ppbw (parts per billion by weight) depending on the source. It is desirable to remove the trace amounts of these metals from crude oils.
Various methods to remove trace metal contaminants in liquid hydrocarbon feed such as mercury have been disclosed. In U.S. Pat. No. 6,350,372 B1, a liquid hydrocarbon feed is mixed with a miscible sulfur compound and then placed in contact with a fixed bed absorbent for removal of at least a portion of the mercury on an elemental basis. U.S. Pat. No. 4,474,896 discloses the use of absorbent compositions, e.g., polysulfide based, for removal of elemental mercury (Hg0) from gaseous and liquid hydrocarbon streams. U.S. Pat. Publication Nos. 2010/0032344 and US2010/0032345 describe processes to remove elemental mercury Hg0 from crude oil consisting of stripping the mercury-contaminated crude with gas in a heated vessel, and then removing the mercury from the stripped gas in an adsorption bed.
There are also a number of commercially available processes and products for the removal of elemental mercury Hg0 from hydrocarbon streams including but not limited to ICI Synetix’ Merespec™ fixed bed absorbents, UOP's HgSIV™ HgSIV™ regenerative mercury removal adsorbents, and Johnson Matthey's Puraspec™ and Puracare™ granulated absorbents for the removal of mercury from naphtha and/or gaseous hydrocarbon streams.
US Patent Application Nos. 2010/0032344 and 2010/0032345 disclose a process for removing elemental mercury concentration with a liquid/gas contactor, with simulations showing 90% mercury removal at a pressure from <1 to −3 Bars and a temperature of greater than 150° C., conditions common at crude oil well sites. It is indicated that the liquid/gas contact is carried out in a vessel that provides direct contact of the treated gas stream with the liquid hydrocarbon stream without contacting any other materials or devices, giving 90% removal rate.
Studies have been conducted to measure mercury levels in crude oil as well as the percentage of mercury in the forms of particles, which can be removed by filtration or centrifugation. It was shown that in crude oils containing more than 50 ppbw mercury, the percent mercury in particles which can be removed by laboratory filtration or centrifugation is over 25% with an average of 73%. It is believed that the remaining 27% mercury is primarily in the form of fine particles. It was also shown that in most samples of crude oils and condensates, the predominant form of mercury is non-volatile, and not in the form of elemental mercury Hg0 which is volatile. It is well known in the art that volatile mercury is readily removed from hydrocarbons upon stripping or sparging with a low mercury gas stream. Quantitative Reitveld XRD analysis of the recovered solids from a crude sample show the only mercury phase to be meta-cinnabar (HgS) and this is assumed to be the predominant mercury species in crude oil.
As adsorption technology does not work well for crude oils and condensates with low levels of mercury, and particularly crude oils containing the non-volatile form of mercury, which has not been well addressed in the prior art. There is a need for improved methods for the removal of mercury from liquid hydrocarbon steams, particularly non-volatile form of mercury.